1. Field of the Invention
The present invention relates to integrated circuits, and more specifically to a CMOS non-volatile memory circuit.
2. Background Art
In the field of data storage, there are two main types of storage elements. The first type is volatile memory that has the information stored in a particular storage element and the information is lost the instant the power is removed from the circuit. The second type is a non-volatile storage element in which the information is preserved even with the power removed. Of the second type, some designs allow multiple programming while other designs allow one-time programming. Typically, the manufacturing techniques used to form non-volatile memory are quite different from a standard logic processes, thereby dramatically increasing the complexity and chip size.
Complimentary Metal Oxide Semiconductor (CMOS) technology is the integration of both NMOS and PMOS transistors on a silicon substrate. The NMOS transistor consists of a N-type doped polysilicon gate, a channel conduction region, and source/drain regions formed by diffusion of N-type dopant in the silicon substrate. The channel region separates the source from the drain in the lateral direction, whereas a layer of dielectric material that prevents electrical current flow separates the polysilicon gate from the channel. Similarly, the architecture is the same for the PMOS transistor but a P-type dopant is used.
The dielectric material separating the polysilicon gate from the channel region, henceforth referred to as the gate oxide, usually consists of the thermally grown silicon dioxide (SiO2) material that leaks very little current through a mechanism called Fowler-Nordheim tunneling under voltage stress. When stressed beyond a critical electrical field (applied voltage divided by the thickness of the oxide), the transistor is destroyed by rupturing of the oxide.
Thin oxides that allow direct tunneling current behave quite differently than thicker oxides, which exhibit Fowler-Nordheim tunneling. Rupturing thin oxide requires consideration for pulse width duration and amplitude to limit power through the gate oxide to produce reliable, low resistance anti-fuse.
Rupturing the gate oxide is a technique used to program a non-volatile memory array. U.S. Pat. No. 6,044,012 discloses a technique for rupturing the gate oxide, but the oxide is about 40 Å to 70 Å thick. The probability of direct tunneling, rather than Fowler-Nordheim tunneling, of gate current through an oxide of this thickness is extremely low. Also, the voltage required to rupture the oxide is substantially higher and requires a charge pump circuit. The '012 patent does not disclose final programmed resistance, but such is believed to be in the high kilo (K) ohms range.
U.S. Pat. No. 5,886,392 discloses a one-time programmable element having a controlled programmed state resistance with multiple anti-fuses. Both the final resistance values are high in the K-ohms range and the spread of these values is wide as well. Again, a complicated circuit would have to be designed if the final resistance is not within a tight range. Adding more anti-fuses can lower the resistance but increases the die size.
What is needed is a one-time programmable CMOS circuit and method that is compatible with non-volatile memory array architecture for sub 0.13 μm process technology.